This application claims the priority benefit of Canadian Patent Application No. 2308092 filed on May 10, 2000 as file no. 30319.34 and entitled Production of Hollow Ceramic Membranes by Electrophoretic Deposition.
The present invention relates to the production of hollow ceramic membranes by electrophoretic deposition. In particular, the present invention relates to the production of small cross-sectional area hollow ceramic membranes by electrophoretic deposition.
It is well known to deposit coatings of material by electrophoretic deposition (xe2x80x9cEPDxe2x80x9d). EPD is a combination of electrophoresis and deposition. Electrophoresis is the movement of charged particles in an electric field. Deposition is the coagulation of particles into a mass.
In U.S. Pat. No. 5,580,835 to Dalzell et al., a process for creating ceramic fibers by EPD is described. The ceramic fibers produced by this process are fully dense, non-porous fibers. The described EPD process uses a colloidal metal hydrate from an aqueous sol where the metal hydroxide particle size is in the range of about 15 nm. The sols are produced by hydrolysis and peptization of an organometallic compound in an aqueous medium. The resulting ceramic fiber is non-porous and filly dense as a result of the small particle size of the sol and the sintering process. Because the sol is aqueous, hydrogen evolution is unavoidable and steps must be taken to minimize hydrogen evolution and to permit hydrogen to escape such that it does not embed in the deposited material. One means of doing so disclosed in this patent is to use a low potential and to continuously move the fiber during the deposition process.
As is apparent in the Dalzell et al. Patent, it is conventionally believed that in order to achieve uniform deposition, only ceramic particles of submicron size may be used in an EPD process. As a result, the resulting ceramic materials, after sintering, are not porous.
It is desirable for certain applications to produce a porous hollow ceramic fibre or membrane. Such fibres may be produced by extruding a mixture of ceramic powder and polymeric binder as disclosed in U.S. Pat. No. 5,707,584. The extruded tube or fibre may then be heat treated to remove the polymeric binder leaving a porous ceramic matrix. The porous ceramic matrix may then be coated by dipping in sols, drying and sintering to add thin layers to the microporous matrix. These are difficult and costly methods. It would be advantageous to have an alternative method of producing porous ceramic fibres or tubes or hollow ceramic membranes.
Therefore, there is a need in the art for a method of producing porous ceramic fibres or hollow ceramic membranes by electrophoretic deposition.
The present invention provides methods for producing hollow ceramic membranes by electrophoretic deposition. The hollow ceramic membranes may have a small cross-sectional area of about 1.0xc3x9710xe2x88x925 mm2 to about 25 mm2. The cross-sectional configuration of the hollow ceramic membranes may be any geometry such as circular, square, rectangular, triangular or polygonal. The hollow ceramic membranes produced by the methods of the present invention may have multiple layers but always the innermost layer, or the first deposited layer is porous and made by electrophoretic deposition. Subsequent layers may be porous or non porous and deposited before or after sintering the first layer. If it is deposited after sintering, it may require additional sintering steps. Additional layers may be deposited by further electrophoretic deposition, sol-gel coating, dip coating, vacuum casting, brushing, spraying or other known techniques.
Therefore, in one aspect of the invention, the invention is a method of producing a porous hollow ceramic membrane comprising the steps of:
(a) providing a suspension of a particulate ceramic material in a non-aqueous liquid;
(b) electrophoretically depositing the particulate material onto an electrically conductive fibre core;
(c) drying the fibre core-bearing the deposited material; and
(d) sintering the fibre core bearing the deposited material at a temperature and for a length of time sufficient to combust the fibre core while producing a porous hollow ceramic membrane.
The fibre core may be a bundle of individual fibres which is infiltrated by the particulate material upon electrophoretic deposition such that upon removal of the fibre core, the membrane comprises a hollow core comprising a plurality of elongate cylindrical pores. Alternatively, the fibre core may be coated with an organic binder to prevent infiltration of the particulate material during electrophoretic deposition.
In one embodiment, the porosity of the membrane may be controlled by controlling the duration and temperature of the sintering step, by controlling the particle size, size distribution and/or the surface area of the ceramic material, by adding sintering additives in the suspension where the additives will deposit concurrently with the ceramic material, by adding a combustible particulate material, such as carbon, carbon black or a suitable organic or polymeric material, to the ceramic material which is concurrently deposited with the ceramic material, wherein said combustible material is removed by combustion during the sintering step.
In one embodiment, the electrophoretic deposition step may be repeated at least once using a ceramic particulate material that is different or has different characteristics such that a multi-layer ceramic hollow membrane where each layer has different characteristics results. The electrophoretic deposition step may be repeated at least once under conditions, as described herein, to produce layers having different porosities.
The non-aqueous liquid may be selected from the group comprising of ethanol, methanol, isopropanol, butanol, acetone, butylamine, acetylacetone methyl ethyl ketone or mixtures thereof.
In another aspect of the invention, the invention is a method of producing a tubular electrode supported electrochemical fuel cell comprising the sequential steps of:
(a) electrophoretically depositing an anodic or cathodic material onto a fibre core to create a porous electrode layer;
(b) depositing a solid electrolyte layer onto the electrode layer; and
(c) drying and sintering the core bearing the deposited anode or cathode layer and the solid electrolyte layer at a temperature and for a length of time sufficient to combust the core and to create a fully dense electrolyte layer while maintaining the porosity of the inner electrode layer;
(d) depositing an outer electrode layer onto the solid electrolyte layer, which is of an anodic material if the inner layer comprises a cathodic material, or a cathodic material if the inner layer comprises an anodic material; and
(e) sintering the end product at a temperature and for a length of time sufficient to bond the outer electrode layer to the solid electrolyte layer while maintaining the porosity of the outer and inner electrode layers.
Preferably, the electrolyte layer is deposited by electrophoretic deposition.
In another aspect of the invention, the invention is a method of producing a tubular electrode supported electrochemical fuel cell comprising the sequential steps of:
(a) electrophoretically depositing an inner electrode layer comprising an anodic or cathodic material onto a fibre core and sintering the core bearing the inner electrode layer at a temperature and for a length of time sufficient to combust the core and partially densify the inner electrode layer while maintaining the porosity of the inner electrode layer;
(b) depositing a solid electrolyte layer onto the electrode layer; and
(c) drying and sintering the core bearing the deposited anode or cathode layer and the solid electrolyte layer at a temperature and for a length of time sufficient to create a fully dense electrolyte layer and bond the electrolyte layer to the inner electrode layer while maintaining the porosity of the inner electrode layer; and
(d) depositing an outer electrode layer onto the solid electrolyte layer, said outer electrode layer comprising an anodic material if the inner layer comprises a cathodic material, or a cathodic material if the inner layer comprises an anodic material; and
(e) sintering the end product at a temperature and for a length of time sufficient to partially densify the outer layer, bond the outer electrode layer to the solid electrolyte layer while maintaining the porosity of the outer and inner electrode layers.
Preferably, the electrolyte layer is electrophoretically deposited onto the inner electrode layer by inserting an electrophoretic electrode within the inner electrode layer. Alternatively, the inner electrode layer is comprised of a cathodic material and is used as the electrophoretic electrode to electrophoretically deposit the electrode layer onto the inner electrode layer.
In yet another aspect of the invention, the invention is a method of producing a tubular electrode supported electrochemical fuel cell comprising the sequential steps of:
(a) providing a porous hollow inner electrode layer comprising an anodic material;
(a) electrophoretically depositing a solid electrolyte layer onto the inner electrode layer by inserting an electrophoretic electrode within the inner electrode layer;
(b) drying and sintering the core bearing the deposited anode or cathode layer and the solid electrolyte layer at a temperature and for a length of time sufficient to create a fully dense electrolyte layer and bond the electrolyte layer to the inner electrode layer while maintaining the porosity of the inner electrode layer; and
(c) depositing an outer electrode layer onto the solid electrolyte layer, said outer electrode layer comprising a cathodic material; and
(d) sintering the end product at a temperature and for a length of time sufficient to partially densify the outer layer, bond the outer electrode layer to the solid electrolyte layer while maintaining the porosity of the outer and inner electrode layers.